The bulk of the raisins produced in the United States are still obtained from hand-picked grapes. Dehydration is accomplished by either "natural" sun drying or in gas-heated hot air tunnels. While the natural method is by far the most common method of making raisins it is not without disadvantages. Considerable hand labor is involved since the bunches of grapes must be turned after several days in order to obtain uniform drying and when drying is complete, the raisins must be transferred from the drying trays for further processing. Also, during drying there is always some loss due to birds, rodents, insects and, most importantly, weather. Since it generally requires several weeks for natural drying, i.e. to achieve a moisture content of about 15%, there is substantial risk of loss due to inclement weather. In 1978, for example, it has been estimated there was approximately 70% weather-related loss of the natural dried raisin crop.
In view of increasing labor costs and the other problems associated with the natural method, there has been renewed interest in forced air drying procedures and methods to accelerate dehydration using such processes. Treatment of the grapes with various materials to increase the drying rate is known. For example, grapes have been treated with hot caustic solutions (0.25% sodium hydroxide dissolved in water) prior to dehydration with hot air. While dipping in a hot caustic solution significantly increases the drying rate, there are certain problems associated with this procedure. The fine cracks which are formed in the skin as a result of such treatment, and which are believed to contribute to the rapid escape of water, often results in excessive stickiness. This is highly objectionable since sticky raisins are difficult to package and, when boxed, form a "lump" from which the individual raisins are not easily separated. Sticky raisins are also objectionable to the consumer.
To overcome the problems associated with such treatment, it has been suggested that hot caustic dip be replaced by a cold dip process. In the cold dip procedure the skins of the grapes are not cracked or checked and the resulting raisins are less sticky and have a more desirable and uniform color. Typically the operation involves immersing the grapes for 3 to 5 minutes in an aqueous dipping solution maintained at ambient temperature and containing about 2.5% alkali metal carbonate, about 2% fatty acid or fatty ester and an emulsifying agent.
General reference to the use of "alkaline" fatty acid ester dipping solutions (hot or cold) to accelerate drying is made at page 307 of Elements of Food Technology, Norman W. Desrosier (Ed.), AVI Publishing Company, Inc. (1977). There is no mention, however, to the conditions or the specific formulations. There is no suggestion in the reference to the utilization of potassium carbonate with fatty acid esters and one skilled in the art would have no reason to expect, based on the reference teachings, that significantly improved results could be obtained therewith.